Method of temperature compensating an electrical apparatus



April 29, 1969 H, P|TT ET AL S METHOD OR TEMPERATURE COMPENSATTNG AN ELECTRICAL APPARATUS Filed July l2, 1965 United States Patent O M METHOD F TEMPERATURE COMPENSATIN G AN ELECTRICAL APPARATUS Howard Pitt, Glendora, and Marshall Cantor, Van Nuys,

Calif., assignors to Physical Sciences Corporation, Ar-

cadia, Calif., a corporation of California Filed July 12, 1965, Ser. No. 471,080 Int. Cl. G01r 3/00 U.S. Cl. 29-595 3 Claims ABSTRACT 0F THE DISCLOSURE This invention relates to a method of progressively heat treating a compensated transducer during construction so as to relieve the stresses in the transducer produced by the various mechanical operations on the transducer. The invention also includes a nal temperature compensation by the use of materials which either exhibit changes in magnetism or eddy current loss with temperature changes so as to produce a compensation for temperature.

This invention relates to transducers. M-ore specifically, the invention relates to compensated transducers which may be used in extreme temperature and radiation conditions. In particular, the compensated transducers constructed in accordance with the invention are more stable under these extreme temperature and radiation conditions than transducers compensated in accordance with prior art methods.

The transducers constructed according to the invention are heat treated in a particular manner so as to Substantially relieve all the stress from the transducer. The heat treated transducers are then temperature compensated in a novel manner so as to completely stabilize the transducer. The prior art transducers either were not heat treated at all, or insuicient methods of heat treating were used. For example, the use of heat treated components to construct the transducer is not suticient since the mechanical assembly of the components by means such as welding induces considerable stress in the transducer. Even subjecting the entire transducer to heat treatment after the transducer has been completely constructed will not remove the stress since the interactions of the various residual stresses present in the transducer components counteract with each other so that stress relief is impossible to accomplish at that late stage. The residual stresses present in the transducer are caused by various mechanical operations that are performed on the transducer as it is `being constructed, and usually the stresses are induced by welding. Unfortunately, the residual stresses are quite important since the magnetic properties of the transducer are varied nonlinearly under extreme temperature conditions by tlie residual stresses.

In accordance with the invention, the transducer is heat treated in a novel way so as to substantially eliminate the residual stresses. Heat treatments are applied to the transducer progressively during the construction of the transducer. For example, each time a weld is made within the transducer `which induces residual stress, the transducer is subjected to a heat treatment to relieve that stress. In this way the stresses produced by the various mechanical operations on the transducer are not allowed to build up to a point Where the stresses cannot lbe relieved by heat treating. After the transducer is completed, it undergoes a final heat treating to substantially relieve all the remaining stresses that may be present within the transducer.

In addition to heat treating as described above, the transducers constructed in accordance with the invention are also temperature compensated in a novel manner. In the prior art transducers, they are temperature compen- 3,440,718 Patented Apr. 29, 1969 sated, if temperature compensated at all, by the use of resistance wire. The resistance wire is connected in series with the coils within the transducer and exhibits an inverse characteristic to the temperature characteristics of the coils. This type of temperature compensation has inherent deficiencies. First, the resistance wire type of temperature compensation provides a loss in the system. Secondly, the resistance wire does not operate with sufcient accuracy over temperature extremes and does not accurately compensate for changes in magnetic flux paths caused by residual stress in the transducer. Moreover, the resistance wire cannot withstand the high radiation conditions which maybe present in the transducers.

In accordance with the invention, the transducers are temperature compensated in a novel manner. The transducers are temperature compensated magnetically instead of through the use of resistance wire. The magnetic compensation is accomplished through the use of materials which either exhibit an increased magnetism or exhibit an increased eddy current loss at one of the temperature extremes. The particular material which is used is dependent upon the particular type of compensation which is necessary for the individual transducer.

The magnetic type of compensation is superior to the Iuse of resistance wire since this type of compensation provides a more linear compensation than could be accomplished by the prior art devices. In addition, the materials used to produce the magnetic type of -compensation are radiation compatible.

The magnetic type of compensation provides only a very limited amount of compensation. This means that the transducers must be heat treated as `described above so as to -remove substantially all of the residual stresses from the transducers. The invention can be looked at as having the large errors corrected by the heat treating and as having the small errors iinally corrected using the materials which exhibit particular magnetic characteristics.

The various 'aspects of the invention, therefore, relate to the method of heat treating by progressively stabilizing the transducer as it is being constructed, and with a linal magnetic compensation of the transducer to eliminate the minor errors which occur when the transducer is being used over a wide temperature and radiation range. Other particular features of the transducer which are considered inventive will be seen by an examination of the drawings, wherein:

FIGURE l is a schematic drawing illustrating a typical electrical conguration for the transducers;

FIGURE 2 illustrates a cross-sectional View of a pressure transducer constructed in accordance with the in- Vention;

FIGURE 3 illustrates a detailed view of a means to connect an aluminum Wire to a stainless steel pin; and

FIGURE 4 illustrates a variable permeance transducer constructed in accordance with the invention.

In FIGURE 1, a pair of identical coils 10 and 12 is connected in series and the pair of coils is connected to three terminals 14, 16- and 18. A movable magnetic core 20 is schematically shown as disposed adjacent to the coils 10 and 12. The core may be moved in 'a longitudinal direction as shown by the arrow 22. If the core is maintained at the midposition, the magnetic properties of the core affect coils 10 and 12 equally and the coils, therefore, exhibit the same inductive characteristics. As the core is moved away from the midposition, one of the coils exhibits an increased inductance and the other coil exhibits a decreased inductance. The relative change in the inductive characteristics of the coils 10 Iand 12 may be measured in any known manner, and this measurement then provides a direct indication of the movement of the core 20. This is a standard type of transducer arrangement and is used for providing an electrical indication of a mechanical movement.-

FIGURE 2` illustrates a rst embodiment of a transducer constructed in accordance with the invention. In FIGURE 2, the transducer is a pressure transducer which produces a mechanical movement in accordance with pressure and a corresponding electrical change in the transducer in accordance with the mechanical movement. The pressure transducer of FIGURE 1 includes a pair of coils 100 and 102 which may be connected in series as shown in FIGURE 1. The coils are wound on a bobbin 104 and the coils are enclosed and separated by three flanges 106, 8 and 110. The coils 100` and 102 may be further protected by the use of insulating material 112 and 114 so that the coils are completely encased. The coils and bobbin 104 are further enclosed by an outer bobbin case 116.

The terminals for the coils and 102 are provided by pins 118 and 120. The end of the bobbin case is sealed by a ring assembly 122. The pins 118 and 120 pass through the ring 122 and are maintained in an insulated position 'by insulating spacers 124 and 126. It is to be appreciated that a third pin would be used for a third terminal, but this pin is not shown since it is behind a center post 128. The post 128 is also maintained in position by insulating spacers 124 and 126, and is slidably engaged with a portion of the bobbin 104. The post provides for a ground connection to the transducer.

The entire transducer of FIGURE 2 is partially enclosed lby an outer casing 130. The outer casing 130A is connected to a second casing 132 which includes a pressure fitting 134. At the left hand side of the transducer of FIGURE 2, a second pressure fitting 136 is connected to the casing 132. A diaphragm mounting ring 138 is disposed on the inner face of the pressure fitting 136, and a diaphragm 140 is attached to the mounting ring 138. The diaphragm 140` is maintained in position by a backup plate 142 which seats within the casing 132. Finally, a diaphragm button 144 is connected to the diaphragm 140 and a core rod 146, which has a core 148 disposed on its end, is connected to the diaphragm button 144.

As can be seen in FIGURE 2, when either pressure or vacuum is introduced to the pressure transducer through the pressure fitting 134 or 136, the pressure or vacuum produces a corresponding mechanical flexing of the diaphragm 140. The flexing of the diaphragm 140` is followed by a corresponding longitudinal movement of the magnetic core 148. Changes in position of the magnetic core 148 change the flux paths within the coils 100` and 102 and produce corresponding changes in the inductive characteristics of the coils. The changes in the inductive characteristics of the coils may be used in a measuring circuit to provide a direct indication of the pressure or vacuum introduced to the fitting 134 or 136.

Although the transducer of FIGURE 2 operates on simple principles, there are difficult problems which arise in constructing the transducer. First, when gross measurements as to pressure or vacuum are made, any small inaccuracies inherent Within the transducer structure are not serious. However, when fine measurements are made, the smallest inaccuracies show up in the inductive characteristics of the transducer. The inaccuracies may be due to many causes, but the effects of the inaccuracies are heightened by temperature and radiation. Most of the inaccuracies in the transducer are d-ue to mechanical stresses which either are introduced into the transducer or part of the structure of the transducer as the transducer is constructed. When extreme temperature or radiation conditions are present, the mechanical stresses affect the magnetic paths within the transducer. The changes in the magnetic paths may sometimes produce effects which are as great as the changes produced by the displacement of the magnetic core 148.

In the prior art, two techniques have been used for reducing mechanical stresses within the transducer. One

technique is to use materials for the transducers which have been stress relieved. The second approach is to heat treat the transducer after the transducer is completed, thereby stress relieving the transducer. Although the above prior art methods are effective in eliminating a large amount of the stresses within the transducer, some residual stresses still remain to produce errors in the characteristics of the transducer. The transducers of the prior art are accordingly compensated, usually by the use of resistance wire in an attempt to further reduce the error in the transducer. However, the residual stresses which remain in the prior art transducers are fairly large and it has been found that even compensating through the use vof resistance wire does not eliminate all of the error from the`transducer. Moreover, the compensating effect of the resistance wire is dissipated when subjected to high radiation, since resistance wire is not radiation compatible.

In the transducer of the present invention, a new technique for heat treating the transducer is used, and the residual stresses present within the transducer are thereby reduced to an extremely low value. Further, any small residual stress present in the transducer after the heat treating is then compensated magnetically by using material which either exhibits increased magnetism or increased eddy current loss with changes in temperature. The magnetic type of compensation is an additional aspect of the invention and is particularly useful for compensating tranducers which are heat treated in accordance with the invention. The magnetic type of compensation is very accurate but can only be used to compensate small errors. Prior art transducers with their large residual errors are not suitable for magnetic compensation since the large error masks out the compensal10n.

In FIGURE 2, compensating discs 150 and 152 are shown at either end of the bobbin 104. These compensating discs are in positions to intercept the magnetic flux paths within the transducer. A final advantage to the use of magnetic compensation is that discs 150 and 152 may be made of material which is radiation compatible. The residual stresses in the transducer of FIGURE 2 are reduced to very low values by progressively heat treating the transducer during its construction. The optimum would be to perform a heat treating operation after each step in the construction of the transducer where residual stress is introduced into the structure of the transducer. For example, in FIGURE 2, dark areas 154 all indicate places where one portion of the transducer is welded to another portion of the transducer. Each time a welding operation is performed in the construction of the transducer, mechanical stress is introduced into the structure of the transducer. Ideally, the transducer would be heat treated after each weld. However, if a larger number of welds are required in the construction of the transducer, heat treating after each weld would be prohibitively expensive. Therefore, as long as the mechanical stresses do not build up too high, it may be possible to perform a heat treating operation after each two or three welds. If, however, the heat treating is postponed until the transducer is completely constructed, sucient mechanical stress will be introduced within the transducer so that it would be impossible at that time to relieve all or most of the stress in a single heat treating operation. Therefore, the invention, as indicated above, includes heat treating the transducer at different times during the construction of the transducer.

A heat treating cycle which would relieve mechanical stress may consist of the following steps: (l) Subjecting the transducer or the then completed portion of the transducer to 1,800 F. for 15 minutes with a subsequent return to room temperature; (2) Subjecting the transducer to -321 F. for 2 minutes with a subsequent return to room temperature; (3) Subjecting the transducer to 1,350 F. for 15 minutes with a subsequent return to room temperature; (4) Subjecting the transducer to 321 F. for 2 minutes with a subsequent return to room temperature; and (5 subjecting the transducer to 900 F. for minutes with a subsequent return to room temperature. The heat treatment as described above will reduce the mechanical stress Within the transducer to almost zero as long as the initial stress is not too great. When the heat treatment is used after each weld or after each two or three welds, the transducer structure is stabilized to have very little residual stress.

After the transducer of FIGURE 2 is heat treated in the desired manner as explained above, the tinal compensation is accomplished using the discs 150 and 152. The compensating discs may either have increased magnetism or increased eddy current loss with changes in temperature, and the particular material is chosen to complement the error in the transducer due to the residual stress. For example, a nickel chromium, referred to as Inconel Alloy X-750 which has the following composition, may be used to provide an increase in the magnetism with decreasing temperature so as to compensate for a loss of magnetism which may be present in the transducer due to the residual stress.

Composition: Percent Nickel1 (min.) 70.0 Chromium l4.0-17.0

Iron 5.0-9.0

Titanium 22S-2.75

Aluminum 0.40-1.00

Columbium2 0.70-1.20 Manganese (max.) 1.0 Silicon (max.) 0.5 Sulfur (max.) 0.01 Copper (max.) 0.5 Car-bon (max.) 0.08

1 Contains a small amount of cobalt.

2 Contains o. small amount of 'tantalum When it is desired to provide an increase in eddy current loss with decreasing temperature, a nickel chromium alloy, referred to as Inconel Alloy 702 which has the following composition, may be used:

Composition: Percent Chromium 14.0-17.0 Iron (max.) 2.0 Carbon (max.) 0.10 Manganese (max.) 1.0 Silicon (max.) 0.7 Sulfur (max.) 0.01 Titanium 0.25-1.00

Aluminum 2.75-3.75 Copper (max.) 0.5 Nickel Remaining The compensation of the transducer by the discs 150 and 152 is accomplished by a trial and error method wherein progressively larger discs are used until the proper size is found. It has been determined that the discs usually run in size from .01 inch to .1 inch in thickness in order to provide the desired compensation.

Since the transducer is designed to be subjected to extreme environmental and radiation conditions, the materials used to contruct the transducer must, of course, be able to withstand the extreme conditions. For example, the coils 100 and 102 are usually made of aluminum wire which has been covered by a hard insulating material. The pins 118 and 120 are usually made of stainless steel. The connection of the aluminu-m wire to the stainless steel pin presents a problem since it is extremely diicult to weld aluminum to stainless steel. The problem has been solved using a novel type of connection as shown in FIGURE 3.

In FIGURE 3, the end of the pin is shown to be cross-slotted at positions 200 and 202 to form four end members 204, 206, 208 and 210. The aluminum wire is brought down for example into the slot 200, wrapped around the pin and then brought through the slot 202. The slots are cut just slightly larger than the diameter of the aluminum wire, and the wire, therefore, ts snugly 'within the slots. The four end members 204, 206, 208 and 210 are then forced together and additionally may be welded to prevent the aluminum wire from escaping from the slots. Moreover, if the diameter of the wire is only slightly smaller than the slots 200 and 202, the forcing together of the four end members 204, 206, 208 and 210 rigidly holds the wire in position. The type of joint described above is valuable since it provides a connection between aluminum and stainless steel without inducing large amounts of stress within the transducer structure.

In FIGURE 4, a variable permeance transducer is sho'wn which is constructed in accordance with the concepts of the invention. For example, the transducer of FIGURE 4 would be heat treated in the same manner as the transducer of FIGURE 2. For example, each time two portions of the transducer are Welded together, or at least after each two or three welds, the transducer is subjected to the heat treatment as disclosed above in order to relieve any stresses which may have been induced in the structure by the weld.

The transducer of FIGURE 4 includes a pair of coils 300 and 302 'which may be connected in series as shown in FIGURE 1. The coils 300 and 302 are supported on a long inner bobbin 304. The inner bobbin 304 extends beyond the ends of the coils and the coils 300 and 302 are supported on the bobbin and maintained in position by three outwardly extending flanges 306, 308 and 310. The coils are sealed at their outer peripheries by insulating material. For example, coil 300 has a coating 312 of insulation, while coil 302 has a coating 314 of insulation. The entire transducer structure is enclosed by an outer casing 316.

The outer casing 316 is sealed to the inner bobbin 304 by a ange 318 which extends between the bobbin 304 and the outer casing 316. The flange 318 and the outer casing 316 are welded together at position 320. As disclosed above, the transducer may be subjected to a heat treatment after each weld so as to relieve any stresses which may be induced into the transducer while making the weld. Electrical connection to the transducer is accomplished through the use of pins 322, 324 and 326. The pins pass through a cup member 328 and are insulated from the cup member 328 by an inner sealing member 330 constructed of insulating material. The pin 326, for example, is disposed through the member 330 which is used to both insulate and seal. The cup member 328 is welded at position 332 to the outer casing 316, and the member 330 is sealed within the cup 328.

The wires from the coils 300 and 302 are attached to the pins 322, 324 and 326 in the same manner as the wires from the coils are attached to the pins in FIG- URE 2. For example, FIGURE 3 illustrates in detail how the wires are attached to *the pins. A magnetic core 334 is disposed within the transducer to affect the inductive characteristics of the coils as the core 33-4 is moved longitudinally within the transducer. The core 334 is attached to a core rod 336 which extends beyond the end of the transducer. The rod 336 has a longitudinal slot 338 which prevents a closed magnetic path from forming around the circumference of the rod. A change in position of rthe core 334 is reiiected directly by a change in the inductive characteristics of the coils and, therefore, the transducer of FIGURE 4 gives a direct reading of the position of the core by a change in the inductive characteristics of the coils.

The core rod 336 is illustrated to have a diameter as large as the diameter of the core itself. This relationship is used for small size transducers so that the core rod is sufficiently large to accurately reflect changes in movement of the rod without errors being introduced due to bending of the core rod. Since the core rod is as large in diameter as the core itself, the use of the slot 338 is important to reduce losses. The transducer of FIG- URE 4 is treated in the same manner as the transducer of FIGURE 2 in that the transducer is heat treated at progressive times during the construction of the transducer. The heat treating of the transducer at progressive times produces a transducer which has extremely low residual stress. The nal compensation of the transducer is accomplished by magnetic means rather than by the prior art method of resistance Wire. The dinal magnetic compensation is produced using discs 340 and/or- 342. The discs 340 and 342 may be made of the same materials as the compensating discs disclosed with reference to the transducer of FIGURE 2. The choice of material for the discs 340 and 342 depends on Whether an increase in eddy current loss or an increase in magnetism is required With decreasing temperature.

It is to be appreciated that the invention has been described With reference to particular embodiments. For example, it is obvious that other types of transducers may be heat treated and magnetically compensated in the same manner as the transducers disclosed in this application. The invention, therefore, may be modified or adapted in many Ways and the invention is only to be limited by the appended claims.

What is claimed is:

1. A method of substantially eliminating the effects of stress lin an electrical apparatus composed of various components, including the steps of:

mechanically yinterconnecting the various components in a plurality of step operations, interrupting the mechanical interconnection of the various components at periodic t-imes after particular ones of the step operations and performing ya heat treatment at each interruption to relieve stresses, Iand magnetically compensating the electric-al apparatus after the l-ast of the heat treatments to substantially eliminate residual effects of stress not eliminated by the heat treatment.

2. A method of substantially eliminating the effects of `stress in an electrical apparatus composed of various components, in-cluding the steps of mechanically interconnecting the various components Iin a plurality of step operations,

interrupting the mechanical interconnection of the various components at periodic times after particular ones of the step operations and performing a heat Itreatment at each interruption to relieve stresses, and

introducing material to the electrical apparatus after the last heat 4treatment to produce an increase in the eddy current loss in the electrical apparatus with decreasing temperatures to compensate for residual efiects of stress in the electrical apparatus.

3. A method of substantially eliminating the effects of stress in an electrical apparatus composed of various components, including the steps of:

mechanically interconnecting the various components in a plurality of step operations,

interrupting the mechanical interconnect-ion of the various components at periodic times -after particular ones of the step operations and performing a heat treatment at each interruption to relieve stresses, and

introducing material to the electrical apparatus after the last heat treatment to produce an increase in the magnetism in the electrical apparatus with decreasing temperatures to compensate for residual eiects of stress in the electrical apparatus.

References Cited UNITED STATES PATENTS 2,473, 1156 `6/ 1949 Littman. '2,599,340 6/1952 Littman et al. 3,308,411 3/1967 Roshala 336-30 JOHN F. CAMPBELL, Primary Examiner.

D. C. REILEY, Assistant Examiner.

U.S. C1. X.`R. 

